I would not wait until 8 months to get my cat fixed, you risk your male cat will start the habit of spraying and your female going into heat. Depending upon your vet and the equipment they are using, they will spay/neuter at 2 lbs, many like to wait until they are at least 4lbs. As for slow to maturing, is your breeder talking about the males, because it is possible that the male's testicles will fall at a later date than other cats. In my foster mutts, they fell at about 3 months (12 weeks) and were neutered at 14 wks (they were between 3 and 4 lbs).